The collisional deactivation of vibrationally highly excited azulene was studied in equimolar supercritical mixtures of xenon and ethane at 385 K from gas-to liquid-phase densities. Azulene with an energy of similar to 20 000 cm(-1) was generated by laser excitation into the S-1 state and subsequent internal conversion to the S-0* ground state. The loss of vibrational energy was monitored by transient absorption at the red edge of the S-3 <-- S-0 absorption band at 290 nm. Transient signals were converted into energy-time profiles using hot band absorption coefficients from shock wave experiments for calibration and accounting for solvent shifts of the spectra. Under all conditions, the energy decays were exponential. At densities below 1 mol/L, the observed collisional deactivation rate constants k(c) of the mixture were equal to the sum of the individual contributions of ethane and xenon collisions as expected from simple gas kinetics. At mixture densities above 2 mol/L, k(c) is smaller than the deactivation rate constant found in neat ethane at half the density. This behavior can be rationalized by an isolated binary collision (IBC) model which relates the collision frequency Z to the radial distribution function g(r) of an attractive hard-sphere particle in a Lennard-Jones fluid. Radial distribution functions obtained by Monte Carlo simulations clearly show that at high densities the less efficient collider xenon preferentially solvates the azulene molecule, reducing the number of azulene-ethane collisions and, therefore, the overall collisional deactivation rate constant with respect to neat ethane solutions.